[makeitdouble]

To extend on jefftk's point, the tests have a specific scope (what the code is responsible for), which probably doesn't cover what it is now expected to do after months/years of getting used.

Your code was supposed to calculate the VAT for a list of purchases, but progressively becomes a way to also force update the VAT cache for each product category and will be called in originally unexpected contexts.

BTW this is the same for comments: they'll cover the original intent and side effects, but not what the method/class is used for or actually does way down the line. In a perfect world these comments are updated as the world around them changes, but in practice that's seldom done except if the internal code changes as well.